The Hair Growth Cycle
It should be explained that in most mammals, hair does not grow continuously, but undergoes a cycle of activity involving alternate periods of growth and rest. The hair growth cycle can be divided into three main stages, namely:
(i) the growth phase known as anagen, during which the hair follicle penetrates deep into the dermis with the cells of the bulb dividing rapidly and differentiating to form the hair, PA1 (ii) the transitional stage known as catagen which is heralded by the cessation of mitosis, and during which the follicle regresses upwards through the dermis and hair growth ceases, PA1 (iii) the resting stage known as telogen in which the regressed follicle contains a small secondary germ with an underlying ball of tightly packed dermal papilla cells. PA1 Yates V M, King C M & Harrap B (1984) "Topical minoxidil in the treatment of alopecia areata". Br. Med. J.288 1087. PA1 Ranchoff R E & Bergfeld W F (1985) "Topical minoxidil reduces blood pressure," J.American Academy of Dermatology, 12 586-587. PA1 Novak E, Franz T J, Headington J T & Wester R C, "Topically applied minoxidil in baldness" (1985) Int J Dermatol 24 82-87. PA1 Franz T J, (1985) "Percutaneous absorption of minoxidil in man". Arch Dermatol, 121 203-206. PA1 Gottlieb T B, Thomas R L & Chidsey C A (1972) "Pharmacokinetic studies of minoxidil", Clin. Pharmacol. Ther., 13, 436-441; and PA1 Thomas R L & Harpootlian H (1975) "Metabolism of minoxidil, a new hypotensive agent. II Biotransformation following oral administration to rats, dogs and monkeys." J. Pharm. Sci., 64, 1366-1371, PA1 R.sup.2 is - H or C.sub.1 to C.sub.10 alkyl PA1 R.sup.3 is - H, C.sub.1 to C.sub.4 alkyl, C.sub.2 to C.sub.4 acyl or SO.sub.3 M; and PA1 M is - H, or a metallic or organic cation.
The initiation of a new anagen phase is revealed by rapid cell proliferation in the germ, expansion of the dermal papilla and elaboration of basement membrane components. The hair cycle is then repeated many times until, as a consequence of the onset of male pattern baldness, most of the hair follicles spend an increasing proportion of their time in the telogen stage, and the hairs produced become finer, shorter and less visible; this is known as terminal to vellus transformation.